From Vietnam biker: Need a help

I'm a biker from Vietnam and I will have a trip to your country (Bangkok) in next month (Sep). I would like to find some where that I can purchase some things for my bike, such as exhaust, light and so on.
Can you let me know where I can got them as cheap and good condition. Where the seller would not cut my neck. ^_^ . I know that in Bangkok there are some big motor market that focus on selling 2nd things. However, I do not know their names.
Look forward to your instruction and thank you in advanced.